It is well known that materials which contain iron oxides, particularly iron ores or iron ore concentrates, are sinterable on sintering machines. During a typical sintering process, the sinterable mixture consisting of the iron ore, recycled material, solid fuel, and fluxes is charged to the sintering machine and the fuel is then ignited on the surface of a charge bed of the machine under an igniting furnace. Thereafter, air is sucked through the bed being sintered and the sintering front proceeds from top to bottom through the bed being sintered. The exhaust gas is sucked into a gas-collecting line from wind boxes disposed under the upper course of the sintering machine, and after being purified is discharged into the atmosphere. In such a sintering process, heat must be transferred from the hot combustion gas to the cold solids. The rates of solids and air are thermally equivalent. The heat exchange requires high air rates and exhaust gas rates. Atmospheric oxygen is consumed only in part. Further, the exhaust gas contains water which has evaporated from the sinterable mixture, in addition to CO.sub.2 produced by the combustion of the fuel and by calcining processes; sulfur oxides produced by the combustion of sulfur, mainly from the added coke or coal; CO produced by an incomplete combustion; and various other gaseous products and infiltrated air, such as air which has flowed into the exhaust gas between the side walls of the pallets and the charge, and has not been utilized.
Dust is also entrained by the exhaust gas. The exhaust gas rate is about 1000 sm.sup.3 (sm.sup.3 =standard cubic meter) per 1000 kg of sinterable mixture or about 1,000,000 sm.sup.3 /h for a sintering machine of 400 m.sup.3.
The sintering process is effected in thin horizontal layers of the charge by the air which is being sucked through the charge, and together with the combustion front proceeds through the charge from top to bottom. The resulting sinter consists of a highly porous material. If the configuration of the sinter is to be preserved, the sintering process must not be changed, as would be the case, e.g., if more than superficial fusion occurred. For this reason a high volume rate of the exhaust gas is one of the requirements for this sintering process.
The exhaust gas rate can be decreased to some extent by a decrease of the rate of unused infiltrated air.
It has also been proposed to recycle the hot exhaust gas from the last suction boxes to the preceding portion of the sintering machine. In this case the exhaust gas rate can be decreased by up to about 40%. See "Stahl und Eisen" 99 (1979), No.7, pages 327-333 and AIME, Iron Making Conference proceedings, Vol. 38, Detroit, Mich., 1979, pages 104-111.
JP-A-52 116 703 discloses a sintering process in which no exhaust gas is discharged into the atmosphere. In this process the oxygen content of the sintering gas is increased by an addition of oxygen before the gas enters the charge bed and all or part of the exhaust gas is fed to a blast furnace. If all of the exhaust gas is fed to the blast furnace, the gas before entering the charge bed must contain more than 30% oxygen and the rate at which the gas is sucked may be as high as 650 sm.sup.3 per 1000 kg sinter, and will decrease as the oxygen content is increased. If only a partial stream of the exhaust gas is fed to the blast furnace and the reminder is recirculated as a sintering exhaust gas, the rate at which gas is sucked also must not exceed 650 sm.sup.' per 1000 kg sinter, and will optimally amount to 500 sm.sup.3 per 1000 kg if the gas contains 17% oxygen. The rate at which gas may be sucked will further decrease as the oxygen content is increased. However, the sintered product obtained by the use of gas at such a permissible rate is poor. Additionally, difficulties are involved due to the connection with the blast furnace on the gas side, and the oxygen consumption is extremely high.
It is therefore an object of the present invention to minimize the rate at which exhaust gas is to be removed in a process of sintering iron oxide-containing materials, and to achieve such a result in an economical process, which also results in a sintered product of high quality.
Further objects of this invention will be readily apparent to those persons generally skilled in the relevant art.